Optical system and optical microscope

ABSTRACT

An optical system which corrects an intensity distribution of incident light to a flat intensity distribution includes: a first lens group which includes at least one lens and has a positive refracting power; a second lens group which includes at least one lens and has a negative refracting power, the second lens group being positioned behind the first lens group in a direction of the incident light; and a third lens group which includes at least one lens and has a positive refracting power, the third lens group being positioned behind the second lens group in the direction of the incident light. In the optical system, the incident light is collimated, and the intensity distribution of the incident light is corrected to the flat intensity distribution by spherical aberrations of the first lens group, the second lens group and the third lens group.

This application claims foreign priority based on Japanese Patentapplication No. 2005-137132, filed May 10, 2005, the content of which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical system for correcting alight intensity distribution, and more particularly to an optical systemfor correcting the intensity distribution of divergent light to auniform distribution.

2. Description of the Related Art

One example of an optical microscope is a confocal microscope. Theconfocal microscope can obtain slice images of a sample without makingthe sample into slice portions, and a correct three-dimensional image ofthe sample can be constructed from the slice images. Therefore, theconfocal microscope is used in, for example, physiological reactionobservation and morphology observation of living cells in the fields ofbiology and biotechnology, or surface observation of an LSI (large-scaleintergration) in the field of semiconductors.

In such a confocal microscope, plural beam spots are produced from laserlight, the sample is irradiated with the beam spots, and the sample isobserved on the basis of fluorescence or reflected light from thesample. In this case, distribution uniformity of the intensity of thelaser light (a Gaussian distribution is obtained with respect to a planeperpendicular to the optical axis) affects the intensities of the beamspots. In order to obtain only a uniform light flux in the vicinity ofthe optical axis of the laser light, therefore, an aperture plate havingan aperture is disposed, and only a light flux which is passed throughthe aperture plate is used.

Also a confocal scanner has been disclosed in which a light intensitydistribution correcting filter is placed between a collimating lens thatconverts divergent light emitted from a fiber end to parallel light, andan aperture plate (for example, see JP-A-2001-228402). The lightintensity distribution correcting filter flattens the intensitydistribution of light which is passed through the aperture of theaperture plate among incident light having a Gaussian light intensitydistribution, and cuts off light other than the light passed through theaperture of the aperture plate.

In a configuration where the light intensity is corrected in this way,the quantity of available light is small, and therefore, in order tosufficiently irradiate a sample, a light source having a correspondinglylarge output power must be used. This causes stray light to beexcessively increased. Therefore, the configuration is not suitable fora case where weak light is handled, such as fluorescence observation.

By contrast, in a configuration where a sample can be irradiated withlight of a uniform intensity without reducing the light quantity, alight intensity uniformalizing lens is used (for example, seeJP-A-11-95109).

FIG. 6 is a diagram of a confocal microscope disclosed in JP-A-11-95109.

Referring to FIG. 6, light emitted from a point light source 61 such asan optical fiber end is converted to parallel light by a collimatinglens 62, the intensity distribution of the parallel light isuniformalized by a light intensity uniformalizing lens 63, and theuniformalized light is incident on a collecting disk 66 through anaperture 65 of an aperture plate 64. The point light source 61 is placedat the front focal point (focal length f) of the collimating lens 62.

A plurality of microlenses (for example, Fresnel lenses) 66 a are formedin the collecting disk 66, and a plurality of pinholes 67 a are spirallyformed in multiple rows in a pinhole disk 67. The collecting disk 66 andthe pinhole disk 67 are coupled to each other so that the pinholes 67 aare located in the respective focal positions of the microlenses 66 a.

Laser light which is incident on the collecting disk 66 is collected bythe microlenses 66 a, and then passed through a beam splitter (notshown) to be collected to the pinholes 67 a. The light which is passedthrough the pinholes 67 a is collected by an objective lens 68, and thenis irradiated on a sample surface 69.

The return light from the sample surface 69 is again passed through theobjective lens 68 and the pinhole disk 67, and then reflected by thebeam splitter (not shown) to enter a camera (not shown) via an imaginglens (not shown). An image of the sample surface 69 is formed on animage receiving surface of the camera.

In this configuration, the collecting disk 66 and the pinhole disk 67are integrally rotated by a member 70, and the sample surface 69 isoptically scanned (raster scanned) by the plural pinholes 67 a, wherebya surface image of the sample surface 69 can be observed through thecamera.

The light intensity uniformalizing lens 63 is a lens which maintains thequantity of the incident light entering from the collimating lens 62,and which uniformalizes the intensity of the incident light (forexample, see JP-A-11-258544).

The light intensity uniformalizing lens 63 is placed between thecollimating lens 62 and the aperture plate 64. The incident lightentering the light intensity uniformalizing lens 63 has a Gaussian lightintensity distribution, so that the intensity of the incident light isstrongest in the vicinity of the optical axis, and the intensity isweaker as further separating from the optical axis. In the lightintensity uniformalizing lens 63, a center portion where the incidentlight is dense is formed as a diffusing lens (concave lens) whichdiffuses parallel light, and a peripheral portion where the incidentlight is not dense is formed as a converging lens (convex lens) whichconverges parallel light. The light intensity uniformalizing lens 63does not cut off light in a portion of the Gaussian distribution wherethe light intensity is low (the peripheral portion of the lens), andhence can maintain about 70 to 90% of the quantity of the incidentlight, thereby preventing a loss of the light quantity from occurring.The light emitted from the light intensity uniformalizing lens 63 isparallel light in which the light intensity distribution issubstantially uniform.

In another laser light intensity distribution-converting optical system,by an afocal optical system of first and second groups(four-lens/two-group configuration) each configured by two lenses andhaving a positive refracting power, the light intensity distribution ofan emission light flux which is parallel light is flattened, and thediameter of a flat distribution region is continuously changed byzooming (for example, see JP-A-3-75612).

In the configuration which is disclosed in JP-A-11-95109, and in whichthe light intensity distribution is uniformalized, however, thededicated lens which converts the light intensity distribution, such asthat disclosed in JP-A-11-258544 is used. Therefore, the configurationis produced with using a dedicated molding die, and a process ofchecking the curvature requires man-hours, resulting in that theconfiguration is expensive. Furthermore, a modulation for coping withthe case of a different NA (numerical aperture) of a light source, and achange of the diameter of output light are hardly conducted. In theabove, the NA is defined as NA=n sin θ where n is the refractive index,and θ is the divergence angle.

In the optical system disclosed in JP-A-3-75612, usual spherical lensesare used, but the four-lens configuration is necessary. Thisconfiguration adversely affects the cost and the space.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and provides a light intensity distribution correcting optical systemand also an optical microscope using the optical system, in which alight intensity distribution of divergent light from a light source isuniformalized and the light use efficiency is improved. Also, in thelight distribution correcting optical system, a difference in NA of thelight source is easily absorbed, and the light distribution correctingoptical system is lower in cost.

In some implementations, an optical system of the invention whichcorrects an intensity distribution of incident light to a flat intensitydistribution, the optical system comprises:

a first lens group which includes at least one lens and has a positiverefracting power;

a second lens group which includes at least one lens and has a negativerefracting power, the second lens group being positioned behind thefirst lens group in a direction of the incident light; and

a third lens group which includes at least one lens and has a positiverefracting power, the third lens group being positioned behind thesecond lens group in the direction of the incident light.

In the optical system of the invention, the incident light iscollimated, and the intensity distribution of the incident light iscorrected to the flat intensity distribution by spherical aberrations ofthe first lens group, the second lens group and the third lens group.

In some implementations, an optical system of the invention whichcorrects an intensity distribution of incident light to a flat intensitydistribution, the incident light having different divergence angles inX-direction and Y-direction of a plane, the optical system comprises:

a first lens group which includes at least one cylindrical lens, has apositive refracting power in a direction of a larger divergence angle ofthe divergence angles, and has no refracting power in a directionperpendicular to the direction of the larger divergence angle; and

a second lens group which includes at least one cylindrical lens, has apositive refracting power in a direction of a smaller divergence angleof the divergence angles, and has no refracting power in a directionperpendicular to the direction of the smaller divergence angle, thesecond lens group being positioned behind the first lens group in adirection of the incident light.

In the optical system of the invention, the incident light iscollimated, and the intensity distribution of the incident light iscorrected to the flat intensity distribution by spherical aberrations ofthe first lens group and the second lens group.

In the optical system of the invention, the first lens group and thesecond lens group are in contact with each other.

In the optical system of the invention, the incident light is laserlight or natural light.

In the optical system of the invention, the intensity distribution ofthe incident light is Gaussian distribution or Airy distribution.

In the optical system of the invention, the incident light is emittedfrom a point light source, and the point light source is an emission endof an optical fiber or a light emitting diode.

In the optical system of the invention, the incident light havingdifferent divergence angles in the X-direction and the Y-direction ofthe plane is emitted from a point light source, and the point lightsource is a laser diode.

In the optical system of the invention, an amount of the sphericalaberrations is substantially equal to or more than 40 percent of a focallength of the first lens group.

In some implementations, an optical microscope of the invention in whicha surface of a sample is irradiated with incident light from a lightsource by an objective lens, the optical microscope comprises:

the optical system of the invention,

wherein the optical system collimates the incident light, and correctsthe intensity distribution of the incident light to the flat intensitydistribution by spherical aberrations of the first lens group and thesecond lens group (and the third lens group), thereby emitting theincident light to the objective lens.

The invention can attain the following effects.

According to the invention, it is possible to realize a light intensitydistribution correcting optical system in which the light intensitydistribution of divergent light from a light source is uniformalized,the light use efficiency is improved, and a difference in NA of thelight source is easily absorbed, and which is lower in cost.

According to the invention, the intensity distribution of divergentlight from a semiconductor laser diode or the like in which NAs inX-direction and Y-direction in a plane are different from each other canbe uniformalized, and the divergent light from the light source can beused at a high efficiency.

According to the invention, it is possible to realize an optical systemwhich requires a small space, and which is easy to handle.

According to the invention, it is possible to realize an opticalmicroscope in which the light intensity distribution of divergent lightfrom a light source is uniformalized, the light use efficiency isimproved, and a difference in NA of the light source is easily absorbed,and which is lower in cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a first embodiment of a light intensitydistribution correcting optical system of the invention.

FIGS. 2A and 2B are diagrams each illustrating a spherical aberration ofa lens.

FIGS. 3A and 3B are views showing effects of a light intensitydistribution correction in a first embodiment.

FIGS. 4A and 4B are diagrams showing a second embodiment of a lightintensity distribution correcting optical system of the invention.

FIGS. 5A and 5B are views showing effects of a light intensitydistribution correction in a second embodiment.

FIG. 6 is a diagram of a confocal microscope disclosed in a related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the invention will be described in detail with reference tothe accompanying drawings. FIG. 1 is a diagram showing a firstembodiment of a light intensity distribution correcting optical systemof the invention. The optical system of the embodiment is placed insteadof the collimating lens and the light intensity uniformalizing lens ofthe confocal microscope described as the related example (FIG. 6).

Referring to FIG. 1, a light source 1 is a point light source such as asemiconductor laser diode (hereinafter, abbreviated as LD), a lightemitting diode (hereinafter, abbreviated as LED), or an end face of anoptical fiber, and emits divergent light. A first convex lens 2 refractsthe divergent light from the light source 1 toward the optical axis bythe positive refracting power to cause the light to be incident on aconcave lens 3 while the beam diameter is reduced. The concave lens 3refracts the light emitted from the first convex lens 2 toward theoutside by the negative refracting power to form substantial parallellight. Because of the spherical aberrations of the lenses, the Gaussiandistribution of the light intensity in the output from the light sourceis converted to a flat light intensity distribution.

A first lens group having a positive refracting power corresponds to thefirst convex lens, a second lens group having a negative refractingpower corresponds to the concave lens, and a third lens group having apositive refracting power corresponds to the second convex lens. Each ofthe lens groups may be configured by plural lenses instead of a singlelens.

This will be described with reference to FIGS. 2A and 2B. FIGS. 2A and2B are diagrams each illustrating a spherical aberration of a lens.

Referring to FIG. 2A, in a convex lens 21, a light flux incident on anouter periphery side of the lens is converged at a focal length f1 whichis in the vicinity of the lens by the spherical aberration, and a lightflux incident on an inner periphery is converged at a focal length f2which is more remote than f1.

Referring to FIG. 2B, in a concave lens 22, by the spherical aberration,a light flux incident on an outer periphery side of the lens is causedto have a larger divergence angle, and a light flux incident on an innerperiphery is caused to have a smaller divergence angle. In FIG. 2B, afocal length f3 or f4 indicates the distance to a point where extensionlines (broken lines) of a light flux diverging when parallel light isincident on the lens converge. Because of the spherical aberration, thefocal length is short (f3) in the outer periphery of the concave lens22, and long (f4) in the inner periphery.

Returning to FIG. 1, in the first convex lens 2, because of thespherical aberration, the light flux in a center portion where the lightintensity is strong is substantially parallel, and that in a peripheralportion where the light intensity is weak is collected to the centerportion.

Since the whole beam diameter is reduced by the first convex lens 2,beams are incident on the inner side of the concave lens 3, and hencethe spherical aberration of the concave lens 3 is weak. Therefore, theconcave lens can convert the whole beams to substantial parallel light,and flatten the light intensity distribution. The second convex lens 4enlarges the reduced beam diameter to enable a zooming operation. In theinvention, the light intensity distribution which is more uniform isrealized by the combination of the spherical aberrations of the firstconvex lens 2, the concave lens 3, and the second convex lens 4. In thiscase, when the first convex lens has a spherical aberration of about 40%or more of the composite focal length, such an effect can be expected.

FIGS. 3A and 3B are views showing effects of the light intensitydistribution correction in the first embodiment.

In FIGS. 3A and 3B, the vertical axis indicates the relative intensityof a beam, and the horizontal axis indicates the beam diameter. Theincident light is divergent light emitted from an end face of an opticalfiber of NA=0.09.

FIG. 3A shows the intensity distribution (Gaussian distribution) of abeam before the light intensity distribution correction. From thefigure, it will be seen that a peak of the intensity is at the center ofthe beam, and the intensity is more attenuated as advancing toward theperiphery.

By contrast, FIG. 3B shows the intensity distribution of a beam afterthe light intensity distribution correction. From the figure, it will beseen that the light intensity is steeply attenuated depending on thedistance a from the center of the beam, but the distribution iscorrected so as to be substantially uniform in a required visual field 2a.

Even when the light intensity distribution is corrected, the value ofshading S indicated by the difference between the beam intensity at thepeak (the center of the beam) and that at a point of the distance a fromthe center is approximately identical.

From this result, the beam intensity distribution is in a state wherethe distribution is flattened within the allowable shading S, and theefficiency of incidence into an aperture (the visual field diameter 2a), which, before correction, is about 22% of the quantity of the lightemitted from the fiber, is about 58% after correction, or improved by2.6 times.

In the embodiment, the first convex lens 2 and the concave lens 3 areclose to each other. Alternatively, a space may be provided between thelenses. When the lenses are bonded together, however, the required spaceis small and the optical system is easy to handle.

The light source is not restricted to an end face of an optical fiberemitting divergent light, and may be another point light source such asan LD (laser diode) or an LED (light emitting diode). Alternatively,natural light may be used.

The intensity distribution of divergent light to be corrected is notrestricted to a Gaussian distribution, and may be an Airy distribution.

As described above, divergent light can be collimated by the threespherical lenses, and the light intensity distribution can be correctedto be flat in a required visual field. Therefore, the cost is very low.Furthermore, the parameters of the lenses are changed in accordance witha difference in NA of a fiber or the like, thereby enabling thedifference in NA to be easily absorbed. Moreover, beam expansion can berealized not by four lenses as in JP-A-3-75612, but by the three lenses.

An optical fiber has the same NA in a plane, but, in an LD, the NA inthe X-direction of a plane is largely different from that in theY-direction. In the configuration of the above-described firstembodiment, it is difficult to uniformalize the intensity distributionof divergent light from such an LD in all directions in a plane. Aconfiguration which can solve the problem will be described withreference to FIGS. 4A and 4B.

FIGS. 4A and 4B are diagrams showing a second embodiment of a lightintensity distribution correcting optical system of the invention. Theoptical system of the embodiment is placed instead of the collimatinglens and the light intensity uniformalizing lens of the confocalmicroscope described as the related example (FIG. 6).

FIG. 4A is a plan view, and FIG. 4B is a side view.

Referring to FIGS. 4A and 4B, a first cylindrical lens 42 has a shortfocal length f5, and a second cylindrical lens 43 has a long focallength f6. The light emission face of an LD 41 is located at a positioncorresponding to the focal lengths of the cylindrical lenses. Thecylindrical lenses are rotated by 90° with respect to each other. Thisconfiguration is employed in order to use a characteristic in whichlight is refracted because a cylindrical lens has a curvature in asectional direction along which the lens can be seen to be semicircular,and light is passed straight through the lens because the lens has nocurvature in a sectional direction along which the lens can be seen tobe rectangular.

Among divergent light from the LD 41, a light flux in a plane where thedivergence angle is large is converted to parallel light by the firstcylindrical lens 42, and than passed straight through the secondcylindrical lens 43 which is rotated by 90°.

By contrast, a light flux in a plane where the divergence angle is small(a plane perpendicular to that where the divergence angle is large) ispassed straight through the first cylindrical lens 42, and thenconverted to parallel light by the second cylindrical lens 43.

At this time, by the spherical aberrations of the cylindrical lenses,the divergent light from the light source having a different emission NAdepending on the plane is converted to have a uniform light intensitydistribution. In this case, when the second cylindrical lens has aspherical aberration of about 40% or more of the focal length f6, suchan effect can be expected.

A first lens group configured by a cylindrical lens corresponds to thefirst cylindrical lens, and a second lens group corresponds to thesecond cylindrical lens.

Each of the lens groups may be configured plural cylindrical lenses inplace of one cylindrical lens.

FIGS. 5A and 5B are views showing effects of the light intensitydistribution correction in the second embodiment.

FIG. 5A shows the intensity distribution before the light intensitydistribution correction, and FIG. 5B shows the intensity distributionafter the light intensity distribution correction. In the figures, theintensity distribution is indicated by curves (horizontal curves) eachconfigured by connecting points of the same intensity.

Because the NAs of the LD in the X- and Y-directions largely differ fromeach other, the beam width in the X-direction is different from that inthe Y-direction. Therefore, the intensity distribution of FIG. 5Ashowing the distribution before the correction, has an oval shape. Thehorizontal curves show that the light intensity distribution is aGaussian distribution.

By contrast, FIG. 5B shows the intensity distribution after thecorrection in which the beam width in the X-direction is equal to thatin the Y-direction. The horizontal curves are dense only in the outerperiphery, and show that the light intensity distribution isuniformalized.

A result that a ratio of light quantities before and after thecorrection is improved by 4.5 times in actual measurement values isobtained.

From the above, divergent light from an LD or the like in which NAs inX- and Y-directions in a plane are largely different can be collimated,the intensity distribution can be uniformalized in a required visualfield, and divergent light from a light source can be used at a highefficiency.

In the embodiment, the intensity distribution of divergent light to becorrected is not restricted to a Gaussian distribution, and may be anAiry distribution.

The invention is not restricted to the embodiments, and includes manychanges and modifications without departing the spirit of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the described preferredembodiments of the present invention without departing from the spiritor scope of the invention. Thus, it is intended that the presentinvention cover all modifications and variations of this inventionconsistent with the scope of the appended claims and their equivalents.

1. An optical system which corrects an intensity distribution ofincident light to a flat intensity distribution, the incident lighthaving different divergence angles in X-direction and Y-direction of aplane, the optical system comprising: a first lens group which includesat least one cylindrical lens, has a positive refracting power in adirection of a larger divergence angle of the divergence angles, and hasno refracting power in a direction perpendicular to the direction of thelarger divergence angle; and a second lens group which includes at leastone cylindrical lens, has a positive refracting power in a direction ofa smaller divergence angle of the divergence angles, and has norefracting power in a direction perpendicular to the direction of thesmaller divergence angle, the second lens group being positioned behindthe first lens group in a direction of the incident light.
 2. Theoptical system according to claim 1, wherein the incident light iscollimated, and the intensity distribution of the incident light iscorrected to the flat intensity distribution by spherical aberrations ofthe first lens group and the second lens group.
 3. The optical systemaccording to claim 1, wherein the incident light is laser light ornatural light.
 4. The optical system according to claim 1, wherein theintensity distribution of the incident light is Gaussian distribution orAiry distribution.
 5. The optical system according to claim 1, whereinthe incident light having different divergence angles in the X-directionand the Y-direction of the plane is emitted from a point light source,and the point light source is a laser diode.
 6. The optical systemaccording to claim 2, wherein an amount of the spherical aberrations issubstantially equal to or more than 40 percent of a focal length of thefirst lens group.
 7. An optical microscope in which a surface of asample is irradiated with incident light from a light source by anobjective lens, the optical microscope comprising: the optical systemaccording to claim 1, wherein the optical system collimates the incidentlight, and corrects the intensity distribution of the incident light tothe flat intensity distribution by spherical aberrations of the firstlens group and the second lens group, thereby emitting the incidentlight to the objective lens.